This invention relates in general to tone detectors. More specifically, the invention relates to tone detectors such as, for example those of the type used in radio receivers to detect a particular tone modulated on a received carrier wave such as used in the addressing of paging radios and the like. More specifically, the invention provides a mixer stage for multiplying a received and carrier demodulated input signal by a reference frequency to determine whether there are any components of the reference frequency in the input signal.
Various known tone detector arrangements are shown in the following patents, the teachings of which are specifically incorporated herein by reference.
U.S. Pat. No. 4,275,271--Soulliard--June 23, 1981 PA0 U.S. Pat. No. 4,258,423--Lane, et al--Mar. 24, 1981 PA0 U.S. Pat. No. 4,142,177--Davis--Feb. 27, 1979 PA0 U.S. Pat. No. 4,047,114--Lane, et al.--Sept. 6, 1977 PA0 U.S. Pat. No. 4,021,653--Sharp et al.--May 3, 1977 PA0 U.S. Pat. No. 3,962,645--Stewart--June 8, 1976 PA0 U.S. Pat. No. 4,052,565--Baxter et al (Oct. 4, 1977) PA0 U.S. Pat. No. 4,047,009--Challen (Sept. 6, 1977) "Walsh Functions: A Digital Fourier Series" by Benjamin Franklin Jacoby, Ph.D., Information Conversion Devices Co., 88 W. Frankfurt St., Columbus, OH 43206.
The above list is intended only to be exemplary of the prior art related to tone detectors and is not intended to be an exhaustive list.
It is known to utilize a sine and/or cosine multiplied input signal to determine the presence or absence of a frequency component of a reference signal in the input signal. Specifically, sine and cosine mixers are used in digital tone detectors for detecting the presence or absence of a specific tone in a signal containing a large number of frequencies. There are many known methods for accomplishing this objective including the use of complex filtering arrangements.
Typically, an input signal to be tone detected is mixed with a pure sine wave signal of the tone frequency to be detected (also referred to as the "reference frequency"). The mixer, in effect, multiplies the input signal by the reference frequency to produce a DC component in the mixed (multiplied) output whenever a frequency component of the input signal is equal to the reference frequency to be detected. The mixer output is low pass filtered and subjected to the transfer function of a square law device to develop a signal proportional to the power of the mixed signal. A popular mixer used in this type of tone detector is a four-quadrant multiplier, such as for example an MC 1595L integrated circuit with appropriate offset adjustment networks. A problem with this known approach is that the tone detector is relatively complex and expensive.
This invention is directed to a mixer stage that can replace the known four-quadrant mixer in a tone detector and to the tone detector circuit including the mixer stage. In essence, a known mixer stage is replaced by a set of transmission gates driven by a Walsh function generator providing either sine or cosine Walsh function coefficient signals to the gates causing them to open and close. The gates effectively multiply an input signal to be tone detected by the sine or cosine function defined by the Walsh function coefficient signals.
A Walsh function generator works by logically breaking down an input signal such as for example a square wave or other signal with recognizable edges into a group of several periodic pulse trains known as Walsh functons. Certain ones of the Walsh functions may be combined in a Walsh weighting and summing network to form a stair-step sine wave. The sine wave frequency is directly related to the frequency of a square wave input to the Walsh function mixer. Similarly, a stair-step cosine wave can be constructed using the remaining Walsh functions. A Walsh weighting and summing network including properly valued resistors separately attenuate each of the Walsh functions to a desired level. These attenuated Walsh functions are then summed to create the desired wave form. In this particular circuit, the Walsh function outputs of the Walsh function generator are used to open and close individual gates of the transmission gates sets. The sine coefficient signals from the Walsh function generator are used to open and close the gates of the "sine" set of transmission gates and the cosine coefficient signals from the Walsh function generator are used to open and close the gates of the "cosine" set of transmission gates.
Walsh function generators per se are known and used in other applications. See for example the following documents, the contents of which are hereby incorporated by reference:
The Baxter '565 patent shows a Walsh function generator used in a digital speech scrambler. The Challen '009 patent teaches how to use a Walsh function generator in a digital tone generator for a radio controlled squelch system. The Jacoby article provides a discussion of the mathematical basis for synthesizing waveforms using Walsh functions.